WHY should we validate our pages?
This is like spellcheck and grammar check for your (X)HTML and CSS
You need to ensure you are following the standards and rules for creating clean markup
Do not use outdated, deprecated features
Accessibility compliance
Ensure the browser is doing what you want

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LINKS
(X)HTML: http://validator.w3.org
CSS: http://jigsaww3.org/css-validator

This is like spellcheck and grammar check for your CSS and (X)HTML.

In the context of (X)HTML, XML, and CSS code, validation is the process of checking a document against the language and grammar (i.e. DTD) it claims to be using. Validation ensures that documents using markup languages (including HTML, XHTML, XML, and CSS) are following the rules of the language as defined by the Word Wide Web Consortium (W3C). A document that passes through the validation process with success -- by being tested with a validator -- is said to be valid HTML, XHTML, XML, or CSS. Valid code is more likely to be displayed consistently and properly in multiple Web browsers, both now and in the future.


Need a DOCTYPE so the browser knows what language to use!